1. Field of the Invention
The present invention relates to a sheet feeding device and an image forming apparatus, and more specifically, to a device that separately feeds a sheet by blowing air toward the sheet.
2. Description of the Related Art
An image forming apparatus such as a copying machine includes a sheet feeding device that feeds sheets to an image forming portion. In such a sheet feeding device, it is required to prevent two or more sheets from being fed to the image forming portion in an overlaid state (double feeding state) so as to prevent the image forming error of the sheet.
In the related art, in order not to feed the sheets to the image forming portion in a double feeding state, a device that detects whether the sheets fed from a sheet feeding portion are double fed has been suggested. As the double feeding detecting device, a configuration that includes an ultrasonic transmitter and an ultrasonic receiver that receives an ultrasonic wave transmitted from the ultrasonic transmitter is known. In this double feeding detecting device, the ultrasonic wave is transmitted from the ultrasonic transmitter to a plurality of locations of the sheet and if the number of locations where an attenuation amount of the ultrasonic wave is larger than a reference value is not less than a predetermined number because the sheets are overlaid, it is determined to be a double feeding state (see Japanese Patent No. 3890766).
The double feeding detecting device is applied to a case where a length of a portion where the sheets are overlaid with each other is short and when an ultrasonic wave is irradiated, the ultrasonic wave is irradiated onto a portion where the sheets are not overlaid and it is not determined as double feeding. Further, the double feeding detecting device may also be used when it is not determined as double feeding because the transmittance of the irradiation position of the ultrasonic wave is not obtained in accordance with the attenuance matching to the double feeding detection due to the unevenness of a fiber of the sheet.
In the meantime, further, an overlay conveying apparatus that conveys the sheets which are separately fed one by one with a predetermined interval by the sheet feeding portion to be partially overlaid is known. The overlay conveying apparatus includes a conveyance belt that continuously conveys the sheets, a delivery belt that receives and conveys a sheet which falls from the downstream of the conveyance belt, and a pressing roller that presses a leading edge of a sheet which is inclined between the pressing roller and the conveyance belt.
In the overlay conveying device, the delivery belt is driven at a sufficiently slow speed with respect to the conveyance belt. By doing this, while a trailing edge hangs on the conveyance belt in a state where the leading edge of a preceding sheet hangs on the delivery belt, a subsequent sheet is conveyed in a state where a leading edge of the subsequent sheet is overlaid with the trailing edge of the preceding sheet (see Japanese Patent Laid-Open No. 11-217148).
However, as disclosed in Japanese Patent Laid-Open No. 11-217148, if the double feeding detecting device disclosed in Japanese Patent No. 3890766 is applied to the overlay conveying apparatus that feeds and conveys the sheet while the trailing edge of the preceding sheet is overlaid with the leading edge of the subsequent sheet, the following problems may be caused. Specifically, when the sheets which are continuously conveyed without a space between the sheets are detected by the double feeding detecting device, it is difficult to discriminate a leading edge of a sheet other than the first sheet and to capture an accurate timing to start the irradiation of the ultrasonic wave.
Here, in the double feeding detecting device, a set overlaid amount is previously known so that a result of irradiating the ultrasonic wave onto a location which is predicted as an overlaid portion is not referred to or the ultrasonic wave is not irradiated onto the location which is predicted as an overlaid portion. However, in this case, due to the variation in the overlaid amount, the number of ultrasonic waves which are actually irradiated on the overlaid portion may be larger or smaller than a predetermined number or the ultrasonic wave is or may be irradiated onto the overlaid portion. Accordingly, the overlaid portion may be erroneously detected as double fed portion.
It is desirable to provide a sheet feeding device and an image forming apparatus that are capable of accurately detecting an abnormality of a feeding state without erroneously detecting a normal feeding state of the sheet to be an abnormal state when a trailing edge of a preceding sheet and a leading edge of a subsequent sheet are fed in an overlaid state.